(a) Technical Field
The present invention relates to an inverter control method and system for an eco-friendly vehicle, and more particularly, to an inverter control method and system for an eco-friendly vehicle, by which overall improvements can be made in terms of switching loss, electromagnetic performance, noise-vibration-harshness (NVH) performance, control stability, etc., when compared to a conventional control methods in which one fixed switching frequency and one fixed sampling frequency are used over the entire operation area.
(b) Background Art
As is well known, eco-friendly vehicles such as pure electric vehicles (EVs), hybrid electric vehicles (HEVs), fuel-cell electric vehicles (FCEVs), etc., use an electric motor as at least one driving source for vehicle driving.
In particular, direct-current (DC) power stored in a main battery of a vehicle is transformed into three-phase alternating-current (AC) power via an inverter between the battery and drive a motor, and a driving force of the motor is transferred to a driving wheel to allow vehicle driving.
In an eco-friendly vehicle, kinetic energy is transformed into electric energy via regenerative braking during deceleration and the electric energy is stored in a battery, and thereafter, while the vehicle driving, the energy stored in the battery is recycled back into driving the motor (e.g., the collected electric energy is recycled as kinetic energy to be utilized by the vehicle to, for example, recharge the battery), thereby improving fuel efficiency.
The motor system, which typically includes a motor, which is operated as a driving source for an eco-friendly vehicle, and an inverter, has several problems associated therewith such as a noise occurring during driving operation/regenerating operation, efficiency degradation caused by switching loss, electromagnetic performance degradation, and so forth.
Generally, if a switching frequency of an inverter increases, noise decreases; as the switching frequency decreases, inverter efficiency and fuel efficiency may be improved.
That is, if the inverter's switching frequency is set to a low fixed frequency (e.g., a base switching frequency is fixed to 4 kHz), electromagnetic performance may be good. However, a significant amount of noise is generated.
When the base switching frequency is set to be high over the entire operational area to reduce the inverter's noise (for example, the base switching frequency is fixed to 8 kHz), NVH performance becomes better (i.e., pulse width modulation (PWM) current ripple is reduced), but electromagnetic performance is deteriorated and switching loss increases (i.e., leading to degradation of heel hold performance in vehicle constraint conditions), such that inverter efficiency and fuel efficiency are degraded as well.
As to electromagnetic performance, as the switching frequency increases, radiated electromagnetic noise increases (e.g., as a result, for example, AM radio reception becomes poor); as the switching frequency decreases, radiated noise decreases and thus electromagnetic performance becomes better.
In a conventional eco-friendly vehicle, to reduce inverter noise which may be sensitively perceived or may displeasing to a driver or a passenger, the inverter's switching frequency is often set high and fixed (e.g., to 8 kHz) and sampling frequency for obtaining information such as sensing current and (estimated) rotor position for controlling the inverter is set equal to the switching frequency (8 kHz) (similarly with the following single sampling scheme).
Herein, a switching frequency (i.e., switching period) may be defined as a period in which ON/OFF of a separate switch in the inverter is repeated once, respectively, and a sampling frequency corresponds to a control period in controlling inverter's current, in which the control period may be defined as a period of repeating rotor position information, a current control operation, duty calculation, and a duty update.
However, in a conventional case, one switching frequency is fixed and used over the entire operation area without any consideration of a motor driving conditions or the like (that is, a fixed frequency scheme is used), resulting in high switching loss caused by heat emission of a switching element and in weakness of electromagnetic performance.
Moreover, when a sampling frequency is high, although inverter control stability becomes better, a load on a processor executing the control increases because the processor has to obtain control parameters such as sensing current, motor angular information, etc., in a shorter period of time and calculate a larger number of control values the processor may become overloaded.
Therefore, it is necessary to control a switching frequency and a sampling frequency according to a driving condition by considering NVH performance, electromagnetic performance, switching loss, control stability, a processor load factor, etc.
That is, in a conventional system, as the switching frequency is set and fixed at a high rate over the entire operational area, there are apparent disadvantages such as electromagnetic performance degradation and switching loss increase as well as some advantages. Consequently, there is a need for a control technique for properly changing a switching frequency according to a driving condition for overall performance improvement and properly adjusting a sampling frequency according to the changed switching frequency.